Concerning variable lead end mills, for example, Japanese Patent Publication No. S63-62323; Japanese Patent Publication No. H05-49408; Japanese Patent Publication No. H07-115254 and Japanese Patent Publication No. H03-19004 disclose this type of end mill, in which a plurality of cutting edges in a helical shape are threadedly located on an outer periphery of a tip portion of an end mill body; at least one or more of helix angles of the cutting edges are different from those of the other cutting edges; and these features make intervals between each cutting edge variable, in the end mill rotating direction, in one or more portions of the end mill body along the direction of its axis.
Regarding this variable lead end mill; in such portion, namely, in the portion where the intervals between each cutting edge are variable in the end mill rotating direction, the periods for raking a work piece by each cutting edge are variable, and thus the helix angles of each cutting edge are variable, too. Therefore, each cutting force of the cutting edges and/or each operating period of them are also variable. These variations prevent the end mill from periodic vibrations occurring in resonance with a natural frequency of the end mill body, i.e., the vibrations caused by the resonance can be prevented from rising.
Moreover, the inventors of the present invention also have disclosed, for example Japanese Unexamined Patent Application Publication No. 2007-268648 (“JP '648”), an end mill in which wall faces of a plurality of chip discharge flutes located on an outer periphery of a tip portion of the end mill body which rotates on its axis, face in an end mill rotating direction cutting edges are formed along the outer peripheral ridge portions of the wall faces the plurality of chip discharge flutes include flute portions which are formed in tiers, and are adjacent to each other in a peripheral direction furthermore, one or more of these chip discharge flutes include flute portions with different tiers from those of the other chip discharge flutes; and, as mentioned in the beginning of this document, the helix angles of each cutting edge in the chip discharge flutes are different from each other.
Meanwhile, as the intervals between each cutting edge of the aforementioned variable lead end mill, are variable; so the periods for raking a work piece with each cutting edge are also variable. Therefore, an amount of chips and/or a thickness of chips, generated by these cutting edges, become varied. Especially the cutting edge with a longer interval to the next cutting edge toward the end mill rotating direction side generates bulkier chips than others, and also the amount of the generated chips increases. Therefore, since this cutting edge has an increased cutting load, compared with other cutting edges, the risk that this cutting edge incurs damages will increase.
In order to control this problem, for example, JP '648 teaches an end mill in which a cutting edge with the aforementioned longer interval in an end mill rotating direction, is connected to a chip discharge flute; and the chip discharge flute includes flute portions with more tiers than the other chip discharge flutes have, and then is secured firmly to get better performance for discharging the many chips generated there.
However, a cross-section perpendicular to the axis of the end mill body, shows that a flute bottom face, namely, fillet, in the chip discharge flute is more convexly curved toward its end mill rotating direction side, compared with a portion where the flute bottom face touches a web thickness circle of the end mill body, i.e., a diameter of the circle is the web thickness of the end mill body. Therefore, this chip discharge flute is not so effective for discharging bulky chips smoothly.
On the other hand, for example, Japanese Unexamined Patent Application Publication No. 2000-52127, Japanese Unexamined Patent Application Publication No. 2001-287114, Japanese Unexamined Patent Application Publication No. 2002-126934, and Japanese Unexamined Patent Application Publication No. 2004-237366, which are not related to variable lead end mills though, disclose end mills; in which a flute bottom face in a chip discharge flute, and/or a secondary flank face adjacent to a cutting edge with the aforementioned longer interval toward an end mill rotating direction, form a center concaved recessed round shape in a cross-section perpendicular to the axis, for getting better discharge performance.
However, since shapes of the intersecting ridgeline portions become sharp, on the flute bottom face and/or on the secondary flank face, where these faces have such center concaved round shapes; the end mills with this shape tend to catch chips and/or tend to incur damage. Also, since the wall thickness, namely, the back metal of this end mill body, on the rear side of a cutting edge in the end mill rotating direction, becomes thin; declines in rigidity and/or strength of the cutting edge are risks that increase.
Therefore, if a flute shape, in which its flute cross-section shape is similar to that of such end mills, is applied to the aforementioned variable lead end mills these result in a decline in a performance for discharging large amounts of chips and/or bulky chips in a chip discharge flute of a cutting edge, and/or cutting edge damages resulting from increasing a cutting load applied to the cutting edge are the risks that increase.
The invention was made under such a background and an object thereof is to provide an aforementioned variable lead end mill in which a cutting edge has sufficient rigidity and strength to prevent it from incurring damages, and an enhanced and reliable performance for discharging large amounts of bulky chips generated by the cutting edge is made possible.